1. Field of the Invention
This invention relates generally to the field of catalyst supports and, more particularly, this invention relates to zirconia-titania-silica tergels useful as supports for olefin polymerization catalysts.
2. Description of the Prior Art
The use of xerogels as catalyst supports is well known. Low pore volume, high surface area ceramic-type supports, while suitable for use in hydrocarbon cracking catalysts, are unsuitable for use in olefin polymerization catalysts, as the activities of such catalysts in polymerization processes are very low.
One form of xerogel which is useful as a polymerization catalyst support is the medium pore volume silica xerogel. Often, the silica xerogel is a two-component gel comprising zirconia or titania incorporated (in minor proportion) in silica.
One type of silica-zirconia xerogel is produced by depositing zirconia on a silica gel. The deposition of zirconia on a silica gel does not provide a support which, when used in a polymerization catalyst, produces polyethylene resins with high melt indexes.
Dombro et al U.S. Pat. No. 4,246,137 (Jan. 20, 1981), the disclosure of which is hereby incorporated by reference, describes a method of preparing high pore volume zirconia-silica catalyst supports by coprecipitation, washing and water removal.
Tergels may also be used as polymerization catalyst supports. Hogan et al U.S. Pat. No. 2,825,721 describes a variety of tergels and their use as polymerization catalyst supports.
It is desirable to produce polyolefin resins which have predetermined properties such as high melt index, broad molecular weight distribution, high weight swell, etc., and attempts have been made to produce such resins using catalysts supported on cogels. For example, Hwang et al U.S. Pat. No. 4,128,500 describes a supported chromium (III) acetylacetonate type catalyst. However, prior catalysts supported on cogels and tergels have not been completely successful in providing control of product polymer characteristics.